You can rebuild the trans to hold more power. Raybestos Blue friction discs, and Koleen (spelling) steel disc should triple the torque capacity of the trans.
You might be able to find different gear sets, and this trans was used in a lot of Jeep applications. My opinion is your probably taking on a project that is going to be expensive, and difficult to complete.
You probably can achieve similar results by getting a higher stall on your torque converter, or changing the final gear in the rear end.
Both are easier, and probably cheaper. Plus if you don't like them it would be easier to go back.
The SC400 auto, and Supra NA auto came with a 4.11:1 rear gear. the SC400 with the same trans has a 3.96:1
I did the torque converter in my 93 sc400 and didn't really like it as it made the car feel heavy to me. Gas mileage with the TC didn't change much as i still had the lock up.
I have a larger gear with a 6-speed now, and i like that much better, but gas mileage stinks.
